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Hawking radiation From Wikipedia, the free encyclopedia

Hawking radiation (also known as Bekenstein-Hawking radiation ) is a with a predicted to be emitted by black holes due to effects. It is named after the physicist , who provided the theoretical argument for its existence in 1974, and sometimes also after the physicist [1] who predicted that black holes should have a finite, non-zero and . Hawking's work followed his visit to in 1973 where Soviet scientists and Alexander Starobinsky showed him that according to the quantum mechanical principle, rotating black holes should create and emit particles. [2] The Hawking radiation process reduces the and the of the and is therefore also known as black hole evaporation .

Because Hawking radiation allows black holes to lose mass and energy, black holes that lose more than they gain through other means are expected to dissipate, shrink, and ultimately vanish. Smaller micro black holes (MBHs) are predicted to be larger net emitters of radiation than larger black holes; thus, they tend to shrink and dissipate faster.

Hawking's analysis became the first convincing insight into a possible theory of quantum . In September 2010, Hawking radiation was claimed to have been observed in a laboratory experiment, however the results remain unrepeated and debated. [3] Other projects have been launched to seek this radiation. In June 2008, NASA launched the GLAST satellite, which will search for the terminal gamma-ray flashes expected from evaporating primordial black holes. In speculative large extra theories, CERN's may be able to create micro black holes and observe their evaporation. [4][5][6][7][8] Contents

 1 Overview  2 Trans-Planckian problem  3 Emission process  4 Black hole evaporation  5 Large extra  6 Deviation from Hawking radiation in loop  7 Experimental observation of Hawking radiation  8 See also  9 Notes  10 Further reading  11 External links

Overview

Black holes are sites of immense gravitational attraction. Classically, the gravitation is so powerful that nothing, not even electromagnetic radiation, can escape from the black hole. It is yet unknown how gravity can be incorporated into , but nevertheless far from the black hole the gravitational effects can be weak enough for calculations to be reliably performed in the framework of in curved . Hawking showed that quantum effects allow black holes to emit exact black body radiation, which is the average thermal radiation emitted by an idealized thermal source known as a black body. The electromagnetic radiation is as if it were emitted by a black body with a temperature that is inversely proportional to the black hole's mass.

Physical insight on the process may be gained by imagining that particle- radiation is emitted from just beyond the . This radiation does not come directly from the black hole itself, but rather is a

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result of virtual particles being "boosted" by the black hole's gravitation into becoming real particles.

A slightly more precise, but still much simplified, view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole whilst the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a (with respect to an observer far away from the black hole). By this process, the black hole loses mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In another model, the process is a quantum tunneling effect, whereby particle-antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon.

An important difference between the black hole radiation as computed by Hawking and thermal radiation emitted from a black body is that the latter is statistical in , and only its average satisfies what is known as Planck's law of black body radiation, while the former fits the data better. Thus thermal radiation contains about the body that emitted it, while Hawking radiation seems to contain no such information, and depends only on the mass, , and of the black hole (the no-hair theorem). This to the black hole information paradox.

However, according to the conjectured gauge-gravity duality (also known as the AdS/CFT correspondence), black holes in certain cases (and perhaps in general) are equivalent to solutions of quantum field theory at a non-zero temperature. This means that no information loss is expected in black holes (since no such loss exists in the quantum field theory), and the radiation emitted by a black hole is probably the usual thermal radiation. If this is correct, then Hawking's original calculation should be corrected, though it is not known how (see below).

As an example, a black hole of one has a temperature of only 60 nanokelvin; in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5 × 10 22 kg (about the mass of the ) would be in equilibrium at 2.7 , absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb, and thereby lose mass. Trans-Planckian problem

The trans-Planckian problem is the observation that Hawking's original calculation requires talking about quantum particles in which the becomes shorter than the Planck near the black hole's horizon. It is due to the peculiar behavior near a gravitational horizon where time stops as measured from far away. A particle emitted from a black hole with a finite , if traced back to the horizon, must have had an infinite frequency there and a trans-Planckian wavelength.

The and the Hawking effect both talk about field modes in the superficially stationary space- time that change frequency relative to other coordinates which are regular across the horizon. This is necessarily so, since to stay outside a horizon requires acceleration which constantly Doppler shifts the modes.

An outgoing Hawking radiated , if the mode is traced back in time, has a frequency which diverges from that which it has at great distance, as it gets closer to the horizon, which requires the wavelength of the photon to "scrunch up" infinitely at the horizon of the black hole. In a maximally extended external Schwarzschild solution, that photon's frequency only stays regular if the mode is extended back into the past region where no observer can go. That region doesn't seem to be observable and is physically suspect, so Hawking used a black hole solution without a past region which forms at a finite time in the past. In that case, the source of all the outgoing can be identified–it is a microscopic point right at the moment that the black hole first formed.

The quantum fluctuations at that tiny point, in Hawking's original calculation, contain all the outgoing

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radiation. The modes that eventually contain the outgoing radiation at long times are redshifted by such a huge amount by their long sojourn next to the event horizon, that they start off as modes with a wavelength much shorter than the . Since the laws of at such short distances are unknown, some find Hawking's original calculation unconvincing. [9][10][11][12][13][14]

The trans-Planckian problem is nowadays mostly considered a mathematical artifact of horizon calculations. [12][15] The same effect occurs for regular matter falling onto a solution. Matter which falls on the white hole accumulates on it, but has no future region into which it can go. Tracing the future of this matter, it is compressed onto the final singular endpoint of the white hole evolution, into a trans-Planckian region. The reason for these types of divergences is that modes which end at the horizon from the point of view of outside coordinates are singular in frequency there. The only way to determine what happens classically is to extend in some other coordinates that cross the horizon.

There exist alternative physical pictures which give the Hawking radiation in which the trans-Planckian problem is addressed. The key point is that similar trans-Planckian problems occur when the modes occupied with Unruh radiation are traced back in time. [16] In the Unruh effect, the magnitude of the temperature can be calculated from ordinary Minkowski field theory, and is not controversial. Emission process

Hawking radiation is required by the Unruh effect and the applied to black hole horizons. Close to the event horizon of a black hole, a local observer must accelerate to keep from falling in. An accelerating observer sees a thermal bath of particles that pop out of the local acceleration horizon, turn around, and free-fall back in. The condition of local thermal equilibrium implies that the consistent extension of this local thermal bath has a finite temperature at , which implies that some of these particles emitted by the horizon are not reabsorbed and become outgoing Hawking radiation. [16]

A Schwarzschild black hole has a metric

The black hole is the background spacetime for a quantum field theory.

The field theory is defined by a local path integral, so if the boundary conditions at the horizon are determined, the state of the field outside will be specified. To find the appropriate boundary conditions, consider a stationary observer just outside the horizon at position r = 2 M + u2 / 2 M. The local metric to lowest order is:

which is Rindler in terms of τ = t / 4 M and ρ = 2u. The metric describes a frame that is accelerating to keep from falling into the black hole. The local acceleration diverges as .

The horizon is not a special boundary, and objects can fall in. So the local observer should feel accelerated in ordinary by the principle of equivalence. The near-horizon observer must see the field excited at a local inverse temperature

,

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the Unruh effect.

The gravitational is by the square root of the time component of the metric. So for the field theory state to consistently extend, there must be a thermal background everywhere with the local temperature redshift-matched to the near horizon temperature:

The inverse temperature redshifted to r' at infinity is

and r is the near-horizon position, near 2 M, so this is really:

So a field theory defined on a black hole background is in a thermal state whose temperature at infinity is:

which can be expressed more cleanly in terms of the of the black hole, the parameter that determines the acceleration of a near-horizon observer.

in with G, c, and k equal to 1, and where κ is the surface gravity of the horizon. So a black hole can only be in equilibrium with a gas of radiation at a finite temperature. Since radiation incident on the black hole is absorbed, the black hole must emit an equal amount to maintain detailed balance . The black hole acts as a perfect blackbody radiating at this temperature.

In engineering units, the radiation from a Schwarzschild black hole is black-body radiation with temperature:

where is the reduced , c is the , kb is the , G is the , and M is the mass of the black hole.

From the black hole temperature, it is straightforward to calculate the black hole entropy. The change in entropy when a quantity of heat dQ is added is:

the heat energy that enters serves increases the total mass:

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.

The radius of a black hole is twice its mass in natural units, so the entropy of a black hole is proportional to its surface area:

.

Assuming that a small black hole has zero entropy, the integration constant is zero. Forming a black hole is the most efficient way to compress mass into a region, and this entropy is also a bound on the information content of any sphere in space time. The form of the result strongly suggests that the physical description of a gravitating theory can be somehow encoded onto a bounding surface. Black hole evaporation

When particles escape, the black hole loses a small amount of its energy and therefore of its mass (mass and energy are related by Einstein's equation E = mc²).

The emitted by a black hole in the form of Hawking radiation can easily be estimated for the simplest case of a nonrotating, non-charged Schwarzschild black hole of mass M. Combining the formulas for the of the black hole, the Stefan-Boltzmann law of black-body radiation, the above formula for the temperature of the radiation, and the formula for the surface area of a sphere (the black hole's event horizon), equation derivation:

Stefan–Boltzmann constant:

Hawking radiation temperature:

Schwarzschild radius:

Schwarzschild sphere surface area of Schwarzschild radius rs:

Stefan–Boltzmann power law:

A black hole is a perfect blackbody:

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Stefan–Boltzmann-Schwarzschild-Hawking black hole radiation power law derivation:

Stefan–Boltzmann-Schwarzschild-Hawking power law:

Where P is the energy outflow, is the reduced Planck constant, c is the speed of light, and G is the gravitational constant. It is worth mentioning that the above formula has not yet been derived in the framework of .

The power in the Hawking radiation from a solar mass ( ) black hole turns out to be a minuscule 9 × 10 −29 watts. It is indeed an extremely good approximation to call such an object 'black'.

Under the assumption of an otherwise empty universe, so that no matter or cosmic microwave background radiation falls into the black hole, it is possible to calculate how long it would take for the black hole to dissipate:

Given that the power of the Hawking radiation is the rate of evaporation energy loss of the black hole:

Since the total energy E of the black hole is related to its mass M by Einstein's mass-energy formula:

We can then equate this to our above expression for the power:

This differential equation is separable, and we can write:

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The black hole's mass is now a function M(t) of time t. Integrating over M from M0 (the initial mass of the black hole) to zero (complete evaporation), and over t from zero to :

The evaporation time of a black hole is proportional to the cube of its mass:

The time that the black hole takes to dissipate is:

Where M0 is the mass of the black hole.

The lower classical quantum limit for mass for this equation is equivalent to the Planck mass, mP. Planck mass quantum black hole Hawking radiation evaporation time:

Where tP is the Planck time. For a black hole of one solar mass ( = 1.98892 × 1030 kg), we get an evaporation time of 2.098 × 10 67 years—much longer than the current at 13.73 ± 0.12 x 10 9 years.

But for a black hole of 10 11 kg, the evaporation time is 2.667 billion years. This is why some astronomers are searching for signs of exploding primordial black holes.

However, since the universe contains the cosmic microwave background radiation , in order for the black hole

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to dissipate, it must have a temperature greater than that of the present-day black-body radiation of the universe of 2.7 K = 2.3 × 10 −4 eV. This implies that M must be less than 0.8% of the mass of the . [17] In common units,

So, for instance, a 1-second-lived black hole has a mass of 2.28 × 10 5 kg, equivalent to an energy of 2.05 × 10 22 J that could be released by 5 × 10 6 megatons of TNT. The initial power is 6.84 × 10 21 W.

Black hole evaporation has several significant consequences:

 Black hole evaporation produces a more consistent view of black hole thermodynamics, by showing how black holes interact thermally with the rest of the universe.  Unlike most objects, a black hole's temperature increases as it radiates away mass. The rate of temperature increase is exponential, with the most likely endpoint being the dissolution of the black hole in a violent burst of gamma rays. A complete description of this dissolution requires a model of quantum gravity, however, as it occurs when the black hole approaches Planck mass and Planck radius.  The simplest models of black hole evaporation to the black hole information paradox. The information content of a black hole appears to be lost when it dissipates, as under these models the Hawking radiation is random (it has no relation to the original information). A number of solutions to this problem have been proposed, including suggestions that Hawking radiation is perturbed to contain the missing information, that the Hawking evaporation leaves some form of remnant particle containing the missing information, and that information is allowed to be lost under these conditions.

Formulae from the previous section are only applicable if laws of gravity are approximately valid all the way down to the Planck scale. In particular, for black holes with below Planck mass (~10 −5 g), they result in unphysical lifetimes below Planck time (~10 −43 s). This is normally seen as an indication that Planck mass is the lower limit on the mass of a black hole.

In the model with large extra dimensions, values of Planck constants can be radically different, and formulas for Hawking radiation have to be modified as well. In particular, the lifetime of a (with radius below the scale of extra dimensions) is given by

where M * is the low energy scale (which could be as low as a few TeV), and n is the number of large extra dimensions. This formula is now consistent with black holes as light as a few TeV, with lifetimes on the order of "new Planck time" ~10 −26 s.

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Deviation from Hawking radiation in

A detailed study of the of a black hole horizon has been made using loop quantum gravity . Loop-quantization reproduces the result for black hole entropy originally discovered by Bekenstein and Hawking. Further, it led to the computation of quantum gravity corrections to the entropy and radiation of black holes.

Based on the fluctuations of the horizon area, a quantum black hole exhibits deviations from the Hawking spectrum that would be observable were x-rays from Hawking radiation of evaporating primordial black holes to be observed. The deviation is such that the Hawking radiation is expected to be centered at a set of discrete and unblended . Experimental observation of Hawking radiation

Under experimentally achievable conditions for gravitational systems this effect is too small to be observed directly. However, a recent experimental set-up created a laboratory "white hole event horizon" that the experimenters claimed was shown to radiate Hawking radiation. [18][19] Some scientists predict that Hawking radiation could be studied by analogy using sonic black holes, in which sound perturbations are analogous to light in a gravitational black hole and the flow of an approximately perfect fluid is analogous to gravity. [20] See also

 Black hole information paradox  Black hole thermodynamics  Trans-Planckian problem  Quantum gravity Notes

1. ^ Charlie Rose: A conversation with Dr. Stephen Hawking & 2. ^ A Brief History of Time , Stephen Hawking, Bantam Books, 1988. 3. ^ Hawking radiation from ultrashort laser pulse filaments Authors: F. Belgiorno, S.L. Cacciatori, M. Clerici, V. Gorini, G. Ortenzi, L. Rizzi, E. Rubino, V.G. Sala, D. Faccio http://arxiv.org/abs/1009.4634 4. ^ S.B. Giddings and S.D. Thomas, "High-energy colliders as black hole factories: The End of short distance physics," arXiv:hep-ph/0106219, Phys. Rev. D65:056010 (2002). 5. ^ S. Dimopoulos and G.L. Landsberg, "Black holes at the LHC", arXiv:hep-ph/0106295, Phys. Rev. Lett. 87:161602 (2001) 6. ^ "CERN courier - The case for mini black holes. Nov 2004 ". http://cerncourier.com/cws/article/cern/29199. 7. ^ American Institute of Physics Bulletin of Physics News, Number 558, September 26, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon 8. ^ Henderson, Mark (September 9, 2008). "Stephen Hawkings 50 bet on the world the universe and the God particle". (London). http://www.timesonline.co.uk/tol/news/uk//article4715761.ece. Retrieved May 4, 2010. 9. ^ Adam D. Helfer: Do black holes radiate? 10. ^ G. 't Hooft, Nuclear Phys B256, 727 (1985) 11. ^ T. Jacobson, Phys Review D 44 1731 (1991) a b 12. ^ http://www.fys.ruu.nl/~wwwthe/lectures/itfuu-0196.ps (p.46) 13. ^ R. Brout, S. Massar, R. Parentani, Ph. Spindel, Hawking radiation without trans-Planckian , Phys. Rev. D 52, 4559 - 4568 (1995) 14. ^ Helfer, Adam D. Trans-Planckian Modes, Back-Reaction, and the Hawking Process , http://arxiv.org/abs/gr- qc/0008016 15. ^ Analogue Gravity a b 16. ^ For an alternative derivation and more detailed discussion of Hawking radiation as a form of Unruh radiation

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see Bryce de Witt's chapter Quantum gravity: the new synthesis pg c.696 in : An Einstein Centenary eds S Hawking and W Israel, ISBN 0521299284 17. ^ [astro-ph/9911309] The Last Eight Minutes of a 18. ^ "First Observation of Hawking Radiation" from the Technology Review 19. ^ Matson, John (Oct. 1 2010). "Artificial event horizon emits laboratory analogue to theoretical black hole radiation". Sci. Am . http://www.scientificamerican.com/article.cfm?id=hawking-radiation. 20. ^ C. Barceló, S. Liberati and M. Visser, “Towards the observation of Hawking radiation in Bose–Einstein condensates,” arXiv:gr-qc/0110036 Int. J. Mod. Phys. A 18, 3735 (2003) . Further reading

 Hawking, S. W. (1974). "Black hole explosions?". Nature 248 (5443): 30. Bibcode 1974Natur.248...30H. doi:10.1038/248030a0. → Hawking's first article on the topic  Page, Don N. (1976). "Particle emission rates from a black hole: Massless particles from an uncharged, nonrotating hole". D 13 (2): 198–206. Bibcode 1976PhRvD..13..198P. doi:10.1103/PhysRevD.13.198. → first detailed studies of the evaporation mechanism  Carr, B. J.; Hawking, S. W. (1974). "Black holes in the early universe". Monthly Notices of the Royal Astronomical Society 168 (2): 399–415. Bibcode 1974MNRAS.168..399C. → links between primordial black holes and the early universe  Barrau, A.; et al. (2002). " from primordial black holes". Astronomy & Astrophysics 388 : 676–687. arXiv:astro-ph/0112486. Bibcode 2002A&A...388..676B. doi:10.1051/0004-6361:20020313. arXiv:astro-ph/0112486.  Barrau, A.; et al. (2003). "Antideuterons as a probe of primordial black holes". Astronomy & Astrophysics 398 : 403–410. arXiv:astro-ph/0207395. Bibcode 2003A&A...398..403B. doi:10.1051/0004-6361:20021588. arXiv:astro-ph/0207395.  Barrau, A.; Féron, C.; Grain, J. (2005). "Astrophysical Production of Microscopic Black Holes in a Low–Planck-Scale World". American Astronomical Society 630 : 1015–1019. arXiv:astro-ph/0505436. Bibcode 2005ApJ...630.1015B. doi:10.1086/432033. arXiv:astro-ph/0505436. → experimental searches for primordial black holes thanks to the emitted antimatter  Barrau, A.; Boudoul, G.. Some aspects of primordial black hole physics . Proceedings of the International Conference on (TH2002). arXiv:astro-ph/0212225. → cosmology with primordial black holes  Barrau, A.; Grain, J.; Alexeyev, S. O. (2004). "Gauss–Bonnet black holes at the LHC: beyond the dimensionality of space". Physics Letters B 584 (1–2): 114–122. arXiv:hep-ph/0311238. Bibcode 2004PhLB..584..114B. doi:10.1016/j.physletb.2004.01.019. arXiv:hep-ph/0311238. → searches for new physics (quantum gravity) with primordial black holes  Kanti, Panagiota (2004). "Black Holes in Theories with Large Extra Dimensions: a Review". International Journal of Modern Physics A 19 (29): 4899–4951. arXiv:hep-ph/0402168. Bibcode 2004IJMPA..19.4899K. doi:10.1142/S0217751X04018324. arXiv:hep-ph/0402168. → evaporating black holes and extra-dimensions  D. Ida, K.-y. Oda & S.C.Park, Phys. Rev. D67 (2003) 064025,Phys. Rev. D71 (2005) 124039,[1]: determination of black hole's life and extra-dimensions  N. Nicolaevici, J. Phys. A: Math. Gen. 36 (2003) 7667-7677 [2]: consistent derivation of the Hawking radiation in the Fulling-Davies mirror model.  L. Smolin, Quantum gravity faces reality, consists of the recent developments and predictions of loop quantum gravity about gravity in small scales including the deviation from Hawking radiation effect by Ansari Spectroscopy of a canonically quantized horizon.  M. Ansari, Area, ladder symmetry, degeneracy and fluctuations of a horizon studies the deviation of a loop quantized black hole from Hawking radiation. A novel observable quantum effect of black hole quantization is introduced.  Stuart L. Shapiro, Saul A. Teukolsky (1983), Black holes, white dwarfs, and neutron stars: The physics of compact objects. p. 366 Wiley-Interscience, Hawking radiation evaporation formula derivation.  Leonhardt, Ulf; Maia, Clovis; Schuetzhold, Ralf (2010). "Focus on Classical and Quantum Analogues for Gravitational Phenomena and Related Effects" . New Journal of Physics .

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http://iopscience.iop.org/1367-2630/focus/Focus%20on%20Classical%20and%20Quantum% 20Analogues%20for%20Gravitational%20Phenomena%20and%20Related%20Effects. External links

 Hawking radiation calculator tool  The case for mini black holes A. Barrau & J. grain explain how the Hawking radiation could be detected at colliders  University of Colorado at Boulder  Hawking radiation on arxiv.org

Hawking radiation observed

 [3] Retrieved from "http://en.wikipedia.org/wiki/Hawking_radiation" Categories: Black holes | Quantum field theory

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